Free-Moment Current Dipoles in Inverse Electrocardiography

A feasibility study is made of the multiple free-moment dipole equivalent cardiac source concept as applied to the inverse problem in electrocardiology. The study deals with a bounded homogeneous model of a dog from which extensive geometry and potential measurements were taken. Torso surface potentials are related to the dipole sources through a set of 268 overdetermined linear algebraic equations. Three source configurations are modeled, 76, 20, and 9 dipoles, and simulation studies are made to evaluate the performance of the models and the feasibility of the concept. Inverse solutions are determined subject to a least-squares error fit of the infinite medium potentials. It is found that the number of free dipoles that can be used as an inverse source model is limited by the solution noise sensitivity, and that the upper limit is in the neighborhood of twenty. The results show that the solutions are unstable in the presence of small geometry perturbations. Localized activation simulated by the 76-dipole source cannot be detected in either the 20- or the 9-dipole solutions, even in the absence of noise and perturbations. Solutions calculated from potentials measured in vivo yield results that are unphysiologic.